1. FIELD OF THE INVENTION
This invention relates to a method of encapsulating a semiconductor. More specifically, the invention relates to a method of encapsulating a semiconductor by using a rapid curable epoxy resin compound which comprises a liquid epoxy resin or a curable liquid mixture comprising an epoxy resin and a diluent, and substantially does not contain a hardener. This invention also relates to a semiconductor device which comprises a semiconductor encapsulated by the above compound.
2. DESCRIPTION OF RELATED ART
Various resins have been used for encapsulating a semiconductor. In particular, solid epoxy resin molding compounds (EMC) have various characteristics which make them suitable as a encapsulating material such as mechanical characteristics, adhesion properties, moisture resistance properties, electric insulating properties and other properties. These materials have been widely used as an encapsulating resin.
However, solid epoxy resin compounds require a kneading machine and a pulverizing machine which are extremely expensive. Additionally, in order to form an encapsulating layer using a molding machine, a tablet with a different size depending on the respective purposes must be prepared. Further, during molding, a portion of the resin cured in the cull and the runner is lost, thereby decreasing the yield.
In order to cure these epoxy resins, a phenol resin-type hardener such as phenol novolak has been generally used. However, in a system in which such a hardener is used, the reaction rate is relatively slow. Some improvement of the reaction rate has been made, but a sufficient reaction rate has not yet been obtained.
As a hardener of an epoxy resin, amino-type compounds such as polyamine and acid anhydride-type compounds have been used. These compounds not only present adverse health effects to humans, but also lower the reaction rate and exert a negative influence on the electric characteristics of an epoxy resin after curing, since the compounds absorb moisture in air even after they are mixed with the epoxy resin. In order to prevent the above problems, an uncured EMC composition comprising a hardener may be sealed in a plastic bag. However, in a system in which a hardener is mixed, the composition must be kept cool for storage and transportation, thereby complicating the handling of the material.
Alternatively, various liquid epoxy resins have been used. It is difficult to impart a sufficient curing rate to a liquid epoxy resin with satisfactory control, whereby an uncured liquid epoxy resin flows out from a gap of a split of mold during molding to form a large amount of flash, or burrs, which results in a reduced yield.
In order to improve operatability of an epoxy resin, a variety of epoxy resin compositions have been proposed in which a special catalyst is used and all components are stored in a single package. A latent type catalyst is known in the art which is a solid and is not dissolved in an epoxy resin at ambient temperature, but can be dissolved in an epoxy resin and causes a curing reaction abruptly when it is heated nearly to a melting point. Examples of these compounds are boron trifluoride, an amine complex, dicyandiamide and derivatives thereof; organic acid hydrazides such as adipic acid dihydrazide; and others. These epoxy resin compounds exhibit excellent storability at ambient temperature. However, the catalyst is not dissolved in the epoxy resin so that it must be dispersed by pasting. Problems concerning conditions of use are limited in that the apparent viscosity of the system is reduced during thermal curing so that dispersion of the catalyst is unstable and is not uniform, and the reaction proceeds slowly. Alternatively, when a latent type of catalyst is used and the reaction rate is increased by heating, an epoxy resin composition generates heat rapidly, thereby causing deterioration and foaming.
Various onuim salts have been used as a latent type catalyst for curing an epoxy resin. For example, in Japanese Provisional Patent Publication No. 126430/1984, a quaternary ammonium salt or a quaternary phosphonium salt, which is a 2-mercaptothiazole, is used as a catalyst in a composition comprising an epoxy resin, a hardener and an inorganic filler. When the quaternary phosphonium salt is used for encapsulating a semiconductor element having a metal electrode, such as aluminum, corrosion of the electrode can be prevented even if it is used under high temperature and high humidity. However, the problem with this system is that a hardener such as a phenol novolak resin, an amine compound, acid anhydride, or a curing accelerator such as imidazoles is used in a combination. Accordingly, the problems of toxicity and others caused by the above hardeners have not been solved. Additionally, curing of an epoxy resin by an onium salt is so rapid that the reaction may be partially non-uniform and large portions of unreacted hardener may remain.
Japanese Provisional/Patent Publication No. 170116/1984, discloses a reaction product of an aromatic diamine and a quaternary phosphonium salt that is used for curing a liquid epoxy resin. However, in order to obtain a completely cured product in this system, it is necessary to use a curing accelerator such as a tertiary amine, imidazoles or metal acetyl acetonate in combination.
Japanese Provisional Patent Publication No. 228745/1984, discloses that an encapsulating epoxy resin of a semiconductor device can be cured by using an aromatic amine adduct and a quaternary phosphonium salt in combination and molded by heating at a temperature of 150.degree. C. for 30 minutes or at 260.degree. C. for 30 seconds. However, this curing rate is not satisfactory. Further, details of the quaternary phosphonium salt have not been described therein.
Japanese Provisional Patent Publications No. 95315/1987 and No. 212418/1987, disclose an epoxy resin composition which comprises a liquid epoxy resin, a specific imidazole compound as a hardener and a phosphonium halide as a latent-type curing accelerator. This composition has storage stability and a rapid curing rate. However, with a gelling time at 130.degree. C. of 4.5/minutes or longer, the curing rate is unsatisfactory for the present application. When this kind of phosphonium salt is not used in combination with an imidazole compound, the curing rate is slower and is not suitable for encapsulating by molding of a liquid epoxy resin. Alternatively, when an imidazole compound is used in the combination, the imidazole compound is a solid and is not dispersed uniformly. As a result, not only is it not dispersed uniformly, but the solid imidazole compound remains, which causes problems in the molding step. In addition, use of an imidazole chemical material lowers the humidity resistance of an epoxy resin encapsulating layer.
Japanese Provisional Patent Publication No. 115427/1991, discloses a latent-type curing catalyst of a solid epoxy resin comprising a diaryliodonium salt of hexafluoroantimonic acid in combination with a radical-generating aromatic compound such as benzopinacol. This combination imparts excellent humidity resistance to a semiconductor device as an encapsulating compound. However, the diaryliodonium salt has poor stability and is difficult to handle.
Japanese Provisional Patent Publication No. 227419/1990, discloses a polymeric compound having a benzylonium salt structure as a side chain which is used as a latent type hardener of an epoxy resin. This system imparts flexibility and impact resistance to a cured resin composition.
Prior art references have disclosed sulfonium salt having a naphthylmethyl group (Japanese Provisional Patent Publication No. 152619/1988), an .alpha.-methylbenzyl group (Japanese Provisional Patent Publication No. 221111/1988) or a 4-alkoxybenzyl group (Japanese Provisional Patent Publication No. 223002/1988). A latent type curing catalyst and such a mechanism of curing an epoxy resin can be applied to an encapsulating material. Also, Japanese Provisional Patent Publication No. 105692/1993, discloses that triphenylbenzylphosphonium hexafluoroantimonate, when used as a latent type curing catalyst of an epoxy resin, does not cause problems of coloring and odor so that it can be applied to an encapsulating material. Further, Japanese provisional Patent Publication No. 96169/1989, discloses that a synthetic method of a benzylpyridinium salt can be used as a latent type curing catalyst of an epoxy resin.